Oscillation generator



July 11, 1939. D. H. RING ET AL OSCILLATION GENERATOR Filed April 22,19:58

/N VE N TORS z aHR/NG By WrW//vrR/NGHAM 5% A TTOR/VEY Patented July 11,1939 UNITED STATES 2,165,509 osoILLA'rioN GENERATOR Douglas H. Ring,Little Silver, and William T. Wintringham, Chatham, N. J., assignors toBell p Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York Application April ,22, 1938,Serial No. 203,558

Claims.

This invention relates to oscillation generators and more particularlyto high frequency generators for use in radio receivers for demodulatingsignal modulated carrier waves.

5 In accordance with the invention, the primary source of theoscillations may be the thermal agitation of free electrons in aresistive impedance or the random fluctuations of the electron currentin the space path of a therminoic vaclO uum tube. It is well known thatresistive conductors or therminoic electron discharge paths behave asthough there were sources of alternating electromotive forces of allfrequencies in a continuous range from zero to at least several 5megacycles per second and of substantially uniform amplitude. We havefound that, by 'amplifying the thermal agitation voltages and sharplyselecting from the amplified oscillations those lying within a verynarrow band of frequencies,

a substantially pure sinusoidal oscillation is obtained having afrequency approximately equal to the mid-frequency of the selected band.The amplitude of the oscillation tends to vary somewhat and in a ratherirregular manner, but by the use of an amplitude limiter or otheramplitude controlling means, the greater part of the variation may besuppressed. Our experiments have indicated that the amplitude variationstend to become slower as the band width of the selecting device isreduced and that'the range of variation, even without amplituderegulation, is not such as to diminish the utility of the generator formany purposes such as the demodulation of high frequency signal waves.

A satisfactory approximation to a sinusoidal oscillation for thedemodulation of speech signals can be obtained by selecting from thesource a band of frequencies of width as great as fty cycles per seco'ndlocated in the superaudible 40 range. Usually a narrower band will bepreferred, but if the width is too small diiliculties may arise from thereduction of the total energy of the selected band. Band widths down toat least ve cycles per second are practicable and provide resultantoscillations that are both purer in wave form and less subject to rapidamplitude variations. The location of the selected band in the frequencyscale is subject only to the practical restrictions pertaining to thedesign and construction of narrow band filters. By the use ofpiezoelectric quartz crystal elements suitable band widths may beobtained at frequencies from thirty kilocycles per second up to severalmegacycles per second.

Particular features of the invention will appear from the followingdetailed description and from the accompanying drawing which illustratesone embodiment and its application in radioreception. Of the drawing,Fig. 1 shows the circuit arrangement of an oscillation generator in ac-5 cordance with the invention and Fig. 2 is a block schematicillustrating the application of the invention in a radio receiver.

Referring to Fig. 1, `resistance R connected be- Y tween the grid andcathode of vacuum tube aml0 plier i0 constitutes a source of thermalagitation electromotive forces from which the ultimate oscillations arederived. Preferably this resistance has a fairly large value, from20,000 ohms to 100,000 ohms and the grid of tube l0 is l5 negativelybiased suiciently to make the fluctuation voltages, Schott effect andthe like, in the plate circuit negligibly small.

To the output terminals of the tube is connected an amplifier Il whichshould have a gain 20 of 50 decibels or more and which may comprisethree or more vacuum tube stages. Following the ampliier is athree-section narrow band piezoelectric crystal F. The filterillustrated is of the ladder type but other well-known con- 25figurations may also be used. The three sections of the lter aresimilar, each comprising a pair of shunt condensers of capacity 'C2 anda series branch constituted by a piezoelectric' quartz crystal X inVseries with a condenser of capacity C1. 30.

'Ihe adjacent shunt capacities are provided by single condensers ofcapacity 2C2`.

The pass band of the filter is llocated adjacent to the principalresonance frequency of the crystal and its width may be controlled byadjust- 35 ment of the values of capacities C1 and C2. By making theseries capacities C1 very small and the shunt capacities C2 large theband width may be made as small as may be desired. Preferably the quartzcrystals are in the form of 40 rectangular plates with their majorsurfaces in the plane ofthe optical and mechanical axes of the crystalas described in U. S. Patent 2,045,991, issued June 30, 1936, to W. P.Mason. With crystal plates of this type the principal reso- 45 nanceoccurs at a fairly low frequency and is well separated from resonancesrepresenting other vibration modes. In an experimental model of theinvention the crystal plates had the dimensions 4559 millimeters in thedirection of the 50 mechanical axis, 18.24. millimeters in the directionof the optical axis, and one millimeter thickness. The principalresonance frequency was 59.966 kilocycles per second and, with values ofcapacities C1 and C2 equal to 200 and 1200 micro 55 microfaradsrespectively, a band Width of about five cycles was obtained. By the useof quartz crystals of the type described, filters with band Widths ofthe above order and with very sharp cut-offs are readily constructed tooperate at frequencies from about 30 kilocycles to 150 kilocycles orgreater,

The output terminals of the lter are connected to the input of anamplifier, the rst stage of which, tube I2, is shown in detail and thesucceeding stages are indicated by block I3. All of the stages may besimilar. Tubes I and I2 are shown as screen-grid tetrodes, but othertubes such as pentodes or triodes may also be used. The gain of thisamplifier need not be large and may be proportioned to provide Whateveroutput energy is desired. The plate current of tube IU and the grid biasof tube I2 are supplied through choke coils I4 and I5 which preferablyshould be of sufficiently large inductance to avoid resonance with thelter capacities at a frequency close to the filter band.

Control of the output amplitude is provided by an automatic volumecontrol circuit comprising a diode rectifier I6 coupled to the outputterminals of amplier I3 and shunted by a resistance Il. The controlvoltage is developed across resistance I'I and is fed back to the grid`of tube I2 and the grids of amplifier I3 through a path including aresistance-capacity timing lter I8. Battery I9, or other equivalentsource, supplies a normal. negative bias to the amplifier grids. Ifdesired, the last stage vacuum tube of amplifier I3 may be operatedunder a condition of space current saturation to provide amplitudelimitation.

Instead of resistance R, vacuum tube il! may be used by itself as asource of voltage fluctuation. For operation in this manner resistance Rmay be reduced to Zero and a positive bias applied to the amplier grid.Under this condition the fluctuations of the electron stream in theplate circuit are emphasized.

The application of the invention to the r-eception of single side-bandsuppressed carrier signals is illustrated by the block schematic diagramof Fig. 2. Single side-band Waves are received on antenna 20 and arereduced in frequency in demodulator 2| by beating with Waves from anadjustable frequency heterodyne oscillator 22. The reduced frequencysingle side-band oscillations are selected by band-pass lter 23 andamplied by amplier 24. From the output of amplier 24 the oscillationspass to a second demodulator 25 in which they are beat with oscillationsfrom the homodyne generator 26 which is of the type shown in Fig. 1. Thedemodulated signal curr-ents are then passed to signal amplifier 21 andto the signal output circuit.

Generator 26 provides a Wave of fixed frequency which for accuratedemodulation of the side-band oscillations impressed on demodulator 25must be such as to coincide with the position of the absent carrierwave. This may be accomplished by the adjustment of the rst beatingoscillator 22. With a band selecting filter of the particular typedescribed in connection with Fig. 1, the frequency of the homodyneoscillations would preferably be between about 30 and 150 kilocycles. Bythe use of other types of piezoelectric crystal selectors higherfrequencies up to several megacycles may be used.

What is claimed is:

1. An oscillation generator comprising a bandpass wave lter having asingle transmission band of Width less than fty cycles per secondlocated in a superaudible frequency range, a high gain amplifierconnected to the input terminals of said lter, and a source of thermalagitation voltages coupled to the input terminals of said amplifier.

2. An oscillation generator in accordance with claim 1 in which the saidsource of thermal agitation voltages is constituted by a metallicresistance element.

3. An oscillation generator in accordance with claim 1 in which the saidsource of thermal agitation voltages is constituted by the electron pathof a thermionic vacuum tube.

4. An oscillation generator in accordance with claim 1 in Which the saidWave lter includes piezoelectric quartz crystals as 4elements determiingthe frequency and Width of the transmission band.

5. An oscillation generator comprising a bandpass Wave filter having asingle transmission band of Width less than fifty cycles per secondlocated in a superaudible frequency range, a high gain amplifierconnected to the input terminals of said lter, a Vsource of thermalagitation voltages coupled to the input terminals of said amplifier, anda second amplifier coupled to the output terminals of said filter, saidsecond amplifier including control means responsive to the outputvoltage thereof for maintaining the amplitude of the output voltage at asubstantially constant level.

DOUGLAS H. RING. WILLIAM T. WINTRINGHAM.

